In the field of surgery, it is often desirable to connect adjacent structures or fragments under conditions in which the connected structures or fragments may ultimately fuse together to form a unitary structure.
As an example, spinal surgical fusion is the process of bringing together two or more vertebrae under conditions whereby the vertebrae fuse together to form a unitary member of the spinal column. Cervical spinal fusion is often prescribed for patients suffering from degenerative disk disease (whose symptoms include neck pain of discogenic origin with degeneration of the disk confirmed by patient history and radiographic studies), trauma (including fractures), tumors, deformity (indicated by kyphosis, lordosis or scoliosis) pseudoarthrosis, and/or failed previous fusions. In carrying out the procedure, the members must be brought together under conditions that are critically controlled to prevent infection, maintain alignment of opposing members, allow for the stress in the bone that is generated as the healing process matures. Immobilization is an important requirement during this healing process.
The process of bone healing has been widely studied in the prior art. Microfractures, once thought to be negative events, are now seen as part of the natural process of bone remodeling and occur within bone in the course of everyday wear and tear.
Such microfractures are healed by ongoing bone remodeling, which occurs in humans in 120 day cycles. In the early stage of these cycles, bone resorption is first accomplished by osteoclasts. This is followed by new bone formation by osteoblasts over the latter part of each cycle.
Osteoblasts serve a critical role in new bone formation, filling in the bony cavity in areas of bone remodeling with bone matrix. The action of osteoblasts is triggered by parathyroid hormone, and is further regulated by thyroxines under the influence of such growth factors as interleukins (1, 6, 11), insulin-like growth factors, and transforming growth factor-B. Osteoblasts are further known to release cytokines to attract osteoclasts.
Osteoclasts serve to release proteases, which act to dissolve bone mineral matrix, collagen, and clear away damaged bone. Osetoclasts also release matrix-bound growth factors and may serve as a chemoattractant for osteoblasts.
Thus, the process of bone healing is now seen as a continual cycle, in which the body's response to microfractures and stress injuries within healing bone actually serve to strengthen healing ultimately and produce more solid bone. Therefore, a plating system to repair bone fractures or to stabilize separate bony structures and allow them to fuse into a single item is enhanced if it can incorporate or harness the elements of the natural bone healing process.
A common approach for the surgical management of cervical disk disease is anterior cervical spinal fusion. The procedure for anterior cervical spinal fusion is initiated by incising a small opening in the front of the neck. There is minimal trauma to the neck tissues. The damaged disk and/or bone spurs are removed anterior to the spinal cord. This approach allows for minimal spinal or cord traction and therefore a quicker recovery period. Frequently, if there is significant spinal cord compression or if there is more than one disk level involved, a small plate is affixed on the anterior surface of the cervical vertebrae to provide greater permanent stability. If a cervical fusion is done, a surgical collar is prescribed to be worn for several weeks following surgery for further stabilization of the neck during recovery.
According to the present art, the plates commonly used for cervical spinal fusion are fabricated from titanium sheets with holes through which screws are inserted to secure the plate to the bodies of neighboring vertebrae. These plates sometimes have an opening in their central portions that enables the surgeon to view the bone graft between neighboring vertebrae (where fusion has been introduced) to ensure that the plate is maintained in alignment as the plate is fixated to the involved vertebrae by screws.
The anterior cervical fusion process as described above has certain inherent risks that endanger success of the operation. It has been recognized that osseous trans-differentiation during the course of bone healing may have a significant role in improving the structural integrity of healed bone following surgery or trauma. The process of bone remodeling following surgery or trauma may, however, result in partial extrusion of the bone plug or screws placed during surgery to fix the position of bones or fragments if the fusion does not take place.
The potential for post-operative screw extrusion has long been a matter of clinical concern. Various locking devices have been described in efforts to hold the surgical screws in place and prevent their extrusion. However, a situation where the surgical screws are, in fact, locked into position may be undesirable. By preventing any degree of post-operative motion by a single screw, mechanical stress may be displaced onto the entire plate and the attached vertebrae, potentially fracturing the plate or allowing the entire plate to become displaced and dysfunctional.
Furthermore, preventing a certain amount of screw transition to occur following implantation impedes trans-differentiation and the formation of stronger new bone within the healing bone. It is therefore deemed desirable according to the present invention to allow at least a certain amount of beneficial post-operative stabilization plate and screw motion, and to prevent a situation where a screw is locked or otherwise firmly secured in its fully-threaded location following insertion.